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Will have to include a set of repeating options to designate metadata sources.
Don't need complete filtering support, but key filters should be exposed with simple properties.
This looks like the most complex "variable" part of the configuration, particularly to support indirect signature verification.
Probably want this to be identical to whatever "simple" format we come up with the IdP since it's common to both.
TrustEngine
Hardwire it, standard engines have no configurable settings anyway.
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